Also called a "blockchain verifier," validators are computers that maintain the blockchain's integrity by constantly computing the linkage from the first block to the last. There is generally an abbreviated version of the blockchain for this purpose that is much smaller than the full chain, which contains every transaction.
Validators for a public blockchain are maintained primarly by volunteers, who typically dedicate a computer to the process. There are thousands of validation nodes for a major blockchain such as Bitcoin or Ethereum. See blockchain
and cryptographic hash function